Mass flow meters for flowing media, which operate according to the Coriolis principle, are known in different designs (cf. e.g. German Patent Specification 41 24 295 and German Offenlegungsschrift 41 43 361 and the publications cited there in each case in column 1, lines 20 to 27, German Patent Specification 42 24 397 and the publications cited there in column 1, lines 23 to 30, as well as German Offenlegungsschrift 196 01 342) and have been increasingly used in practice for some time.
In the case of mass flow meters for flowing media, which operate according to the Coriolis principle, basically there are, on the one hand, those whose Coriolis measuring tube is made at least essentially straight, as a rule exactly straight, and, on the other hand, those whose Coriolis measuring tube is made loop-shaped. In addition, in the case of the mass flow meters under discussion there are, on the one hand, those which have only one Coriolis measuring tube and, on the other hand, those which have two Coriolis measuring tubes. In the case of the embodiments with two Coriolis measuring tubes, they can be hydraulically in series or parallel to one another.
Mass flow meters of the type in question, in the case of which the Coriolis measuring tube is, or the Coriolis measuring tubes are, made straight, with respect to their mechanical construction are simple and can consequently be produced at relatively low cost. In this case, it is also possible to finish or polish the inner surfaces of the Coriolis measuring tube or Coriolis measuring tubes well; they can be polished easily. In addition, they have a relatively low pressure loss. In the case of mass flow meters which operate according to the Coriolis principle, and in the case of which the Coriolis measuring tube is made straight, or the Coriolis measuring tubes are made straight, it can be disadvantageous that both thermally caused expansions or stresses, as well as forces and moments acting from outside, can lead to measurement errors and to mechanical damage, namely stress cracks.
The experts have already dealt with the above-mentioned problems in mass flow meters with straight Coriolis measuring tubes (cf. in particular German Patent Specification 41 24 295, German Offenlegungsschrift 41 43 361, and German Patent Specification 42 24 379). Altogether, a mass flow meter operating according to the Coriolis principle, with a straight Coriolis measuring tube, which has a measurement error of only about 0.1% (cf. the prospectus "Zulassung des Corimass G-Gerates zum eichpflichtigen Verkehr" of the KROHNE Me.beta.technk GmbH & Co. KG), was made successfully.
Mass flow meters operating according to the Coriolis principle, which have only one straight Coriolis measuring tube, have considerable advantages as compared with those mass flow meters which have either two straight Coriolis measuring tubes or one loop-shaped Coriolis measuring tube. The advantage as compared with mass flow meters with two straight Coriolis measuring tubes in particular is to be seen in the fact that flow separators or flow combiners, which are required in the case of mass flow meters with two Coriolis measuring tubes, are not needed. The advantage as compared with flow meters with one loop-shaped Coriolis measuring tube, or with two loop-shaped Coriolis measuring tubes, in particular is to be seen in the fact that a straight Coriolis measuring tube can be easier to produce than a loop-shaped Coriolis measuring tube, that the pressure drop in the case of a straight Coriolis measuring tube is less than in the case of a loop-shaped Coriolis measuring tube, and that a straight Coriolis measuring tube can be cleaned better than a loop-shaped Coriolis measuring tube.
However, mass flow meters which operate according to the Coriolis principle and have one straight Coriolis measuring tube, also have a physically, or mechanically, predetermined disadvantage (cf. European Offenlegungsschrift 0 521 439).
The mass flow meters operating according to the Coriolis principle require that the Coriolis measuring tube be put into oscillation, and in particular by means of at least one oscillation generator; the Coriolis forces, or the Coriolis oscillations, do indeed result from the fact that the Coriolis measuring tube oscillates and from the flowing of mass through the Coriolis measuring tube.
In the case of mass flow meters with two straight Coriolis measuring tubes, or with one loop-shaped Coriolis measuring tube, or with two loop-shaped Coriolis measuring tubes, the Coriolis measuring tubes, or the parts of the loop-shaped Coriolis measuring tubes causing oscillation, are designed identically and, as a rule, located and excited into oscillation so that they oscillate opposite one another. This has the positive consequence that the oscillating system as a whole is not acting as such outwards. The position of the center of mass remains constant and forces which appear are compensated. Consequently, no forces and no oscillations are introduced into the pipeline system in which this mass flow meter is installed, and forces and oscillations of the pipeline system do not influence the measurement result.
In the case of mass flow meters operating according to the Coriolis principle, which have only one straight Coriolis measuring tube, the positive consequence of the Coriolis measuring tubes oscillating opposite one another, explained above, naturally does not occur. The center of mass does not remain constant and forces which appear are not compensated. The consequence of this is, on the one hand, that forces and oscillations are transferred into the pipeline system in which a mass flow meter is installed, and, on the other, that forces and oscillations acting on the mass flow meter, derived from the pipeline system, can influence the measurement result.
The known mass flow meters operating according to the Coriolis principle, explained in detail previously, are indeed directly suitable today for measuring the flow of liquids with a high measurement accuracy, namely with a measurement accuracy of 0.1%. However, they are not suited for the measurement of the flow of gases in the same way. Consequently, the object of the invention is to provide a mass flow meter of the type under consideration, which can also measure the flow of gases with a high measurement accuracy.